(a) Field of the Invention
The present invention relates to a semiconductor device using a Group II-VI compound semiconductor, and more particularly it pertains to a light-emitting diode using a ZnSe compound semiconductor crystal.
(b) Description of the Prior Art
Group II-VI compound semiconductor crystals (hereinafter to be referred to briefly as Group II-VI crystals) are crystals whose carrier transition is of the direct transition type, and there are several kinds of Group II-VI compound semiconductor crystals having an energy band gap (E.sub.g) greater than that of Group III-V compound semiconductor crystals.
Among Group II-VI crystals, especially ZnS (E.sub.g =3.6 eV), ZnSe (E.sub.g =2.67 eV) and CdS (E.sub.g =2.5 eV) have an energy band gap greater than that of GaP (E.sub.g =2.25 eV) which has an energy band gap greater than that of Group III-V compound semiconductor crystals (hereinafter to be referred to briefly as Group III-V crystals).
Accordingly, if a pn junction diode can be made using a ZnS, ZnSe or CdS crystal to fabricate an LED, there may be expected an LED which emits light in a wavelength region shorter than the LED fabricated with a GaP crystal.
Most of the Group II-VI crystals obtained using the prior art crystal growth methods which mostly employ the so-called melt growth technique, however, are of the n type, excluding ZnTe crystal. It is the present state of the art that no control of the conductivity type of these crystals can be made, much less controlling their carrier concentrations. As such, the outstanding features of these Group II-VI crystals have not been fully realized.
Hereunder will be explained the reasons why crystals of the p type conductivity cannot be obtained in these Group II-VI crystals.
When a comparison is made between Zn atoms and Se atoms, which are the two elements constituting a ZnSe crystal, it is noted that both elements have high vapor pressures, and also that Se atoms have a vapor pressure which is about one order higher than that of Zn atoms at a same temperature. Accordingly, when a crystal of ZnSe is grown by the conventional methods, deviation from stoichiometric composition in the grown ZnSe crystal can easily develop. The pattern of this deviation assumes the tendency such that Se atoms having a higher vapor pressure escape out of the crystal to cause a shortage of Se atoms in the crystal. This tendency appears more intensively for higher growth temperatures as those represented typically by the Bridgman method.
Se vacancies which are generated due to the shortage of Se atoms in the ZnSe crystal form a donor level within the crystal, so that most of the crystals grown present by the conventional method shows n type conductivity. As stated above, it will be noted that n type crystals can be obtained either in the form of their natural occurrence, or by intentionally doping an n type impurity. If, therefore, one intends to obtain merely n type crystals, this purpose can be achieved with relative ease. In contrast thereto, it is very difficult to obtain a p type ZnSe crystal. This fact and its reasons will be described hereunder.
Even when atoms of an impurity serving as an acceptor are doped into the crystal in order to provide a crystal of the p type, or more concretely even by doping a p type impurity during the growth of a crystal, or by doping a p type impurity into an n type crystal by relying on the diffusion technique with the intent to alter a portion of this crystal into the p type, or by relying on the alloying technique, the n type crystal will still remain an n type crystal, or even when the crystal has turned successfully into a p type crystal, the resulting crystal will have a very high resistivity, and thus practically no useful p type region could have been obtained in the past.
The reasons therefor are as follows. When an acceptor impurity is doped into a crystal in order to obtain a p type crystal, there will generate, within the crystal, defects which are mainly Se vacancies and which serve as a donor, in accordance with the amount of the acceptor impurity doped, as a natural trend of the crystal to become thermodynamically stable. Thus, the acceptor carriers are compensated, which is called the self-compensation effect.
The defects within the crystal which act as a donor are comprised mostly of Se vacancies as stated above, and in addition these defects are considered to also represent a complex of these Se vacancies and the impurity atoms.
As stated above, with such a prior crystal growth method in which no consideration has been paid to the matter of deviation from the stoichiometric composition of the crystal, it has been extremely difficult to obtain a Group II-VI crystal with a controlled conductivity type, especially p type while satisfying practical purposes. Thus, the state of art may be said to be that it has been technically impossible to obtain a functional device having a pn junction from a Group II-VI compound semiconductor material represented typically by ZnSe crystal.